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Abstract

In this paper, we propose a self-assembled microlens on
top of light-emitting diodes (LEDs) based on hydrophilic effect
under ultraviolet (UV)/ozone treatment to improve extraction efficiency and
to increase viewing angle. The LED chip was encapsulated by polydimethylsiloxane
(PDMS) $(n=1.44)$ before the fabrication process of microlens. The microlens was
made of transparent negative photoresist SU-8 $(n=1.63)$. The SU-8 photoresist became
more hydrophilic after UV/ozone treatment. After hydrophilic zones were produced
by using shadow masks and UV/ozone, the substrate with hydrophilic zones was
dipped in and out of diluted SU-8 photoresist solution with slow and constant
velocity. Meanwhile, a 200-$\mu{\hbox{m}}$ microlens was formed by self-surface tension and cohesion of diluted
SU-8. Finally, microlens was hardened after UV curing. This approach is cost
effective and low time-consuming. It did not require photo masks, heating,
or etch-transfer processes. It's a new fabrication method that can be easily
applied to opto-electronic devices. Integrated with LED, microlens can improve
extraction efficiency 28% in total power and increase viewing angle 17 degree.

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